A description of the prior art concerning high-performance electronically commutated electric motors is cited by Hendershot in “Design of Brushless Permanent Magnet Motors”. According to this, an electronically commutated electric motor consists of the following characteristic components:
1. Stationary stator and rotor in a radial flux design
The stator consists, for example, of a laminated iron ring and a winding mostly made up of three phases. The laminations of the iron ring are subdivided into the characteristic parts of tooth, hammer and back iron yoke. The winding is inserted in the area enclosed by the tooth, hammer and back iron yoke. The area in which the winding is inserted is called a “slot”. The iron ring can be designed to suit various slot configurations.
The rotor consists of a back iron yoke and the permanent magnet generating the flux. The permanent magnet can be composed of several segments; it is preferably made from a single piece and n-pole radially or diametrically magnetized. The number of poles (P) corresponds to the number of magnetized pole areas with alternating polarity. The rotor can be designed as an inner or outer rotor motor. For outer rotor motors, the stator is designed with the back iron yoke located inside and the tooth and hammer pointing towards the outside. For inner rotor motors the stator is designed with the back iron yoke on the outside and the tooth and hammer pointing towards the inside. The magnet on the rotor of an outer rotor motor is located inside and the back iron yoke outside. The magnet on the rotor of an inner rotor motor is located outside and the back iron yoke inside.
This motor design is based on the radial flux principle; i.e. the magnetic flux penetrates the coils in relation to the permanent magnet and its rotational axis in an essentially radial direction. The required offset in the phases of the motor depends on the number of slots, poles and phases and is set by placing the phase windings in different areas of the stator.
2. Commutation device
The motor should be designed with a commutation device which, dependent on the position of the rotor, selects the energizing pattern for the coils that generates maximum torque. The commutation device mostly takes the form of Hall position sensors in combination with a sensor magnet and a MOSFET power amplifier. Here, the Hall position sensors detect the momentary position of the rotor and trigger the MOSFET power amplifier in the required manner.
For electronically commutated motors of the described construction, many different combinations of numbers of poles (2P) and of slots (n) are known. Through the choice of the number of poles and slots, the motor can be adapted to various requirements, such as a trapezoid torque waveform, sinoid torque waveform, low detent torque etc. For especially low detent torque a cant of 2π/n for the pole transitions between the magnets is frequently suggested.
The main disadvantages of the standard motor design for electronically commutated motors lie in the costly manufacturing processes to fabricate the windings for inner rotor motors, and the lack of a stationary housing and the high costs of magnets for outer rotor motors.
3. Stator and rotor in a transversal flux design
For each motor phase in a transversal flux machine, a rotor section with a permanent magnet, or with a permanent magnet section, is provided that has magnetized, alternating poles. The stator has flux guide units with claws to redirect the radial magnetic flux into a transversal magnetic flux, whereby the claws extend parallel to the rotational axis of the permanent magnet net and in the vicinity of the magnetic pole surfaces of the permanent magnet. Each flux guide unit belonging to a phase of the transversal flux machine encloses a concentrically wound toroid coil which, in relation to the permanent magnets, is essentially enclosed by the magnetic flux in a longitudinal direction. For each multiphase transversal flux machine, several such rotor/stator units are arranged next to each other on one axis or stacked one on top of the other, whereby the required offset of the phases is achieved by using several permanent magnets or magnet sections which are offset in relation to each other, or by stacking the stator units with the appropriate offset in the angle of rotation. Each phase thus has its own pole or armature system having a rotor with a permanent magnet, a stator and a dedicated toroid coil. The number of flux collectors or claws corresponds to the number of poles.
An example of a known transversal flux machine with further references to the prior art can be found in DE 198 18 035 A1.
Although the transversal flux design has the advantage of a simpler winding technique for the coils, it requires a higher magnetic volume compared to the radial flux machine to generate a comparable magnetic flux. Moreover, the transversal flux machine has the disadvantage that the number of magnetic poles has to correspond to the number of flux collectors or claws on the flux guide units which goes to restrict the means of influencing the torque waveform for the transversal flux machine.
U.S. Pat. No. 5,854,526 describes a direct current motor having a multi-pole permanent magnet and several flux guide units. Each flux guide unit has a flux collector section and flux concentrator section, whereby each flux guide unit picks up a radial magnetic flux from the permanent magnet and redirects it into a transversal magnetic flux. In addition, the motor has several coils arranged axially parallel to the permanent magnet. The purpose of the motor design revealed in this patent is to provide a low-cost DC motor with a large torque and good performance which can be manufactured with precision.
DE 1 018 142 describes a self-running synchronous motor with two coaxially attached coils that have serrate pole plates made of ferromagnetic material at their ends featuring an approximately even number of annular teeth. The teeth of two pole plates belonging to different coils are combined together to form one tooth. The purpose of this arrangement is to increase the number of pole divisions per unit of length.
The object of the present invention is to provide a new principle for an electronically commutated electric motor which has coils that are easy to wind, which can achieve a comparable performance in relation to known radial flux machines and which allows any desired combination of the number of poles of the permanent magnet and the number of slots of the stator to enable the torque waveform to be influenced according to requirements.